Major League Baseball (MLB) postponed the remainder of a series between the Washington Nationals and New York Mets on Friday due to COVID-19, an early sign of trouble as the league navigates a new season amid the pandemic.

The Nationals' home opener against the Mets was put on hold Thursday, a day after Washington reported one player had tested positive for COVID-19. Later on Thursday, the 2019 World Series champions said at least two additional individuals had come back with positive tests.

MLB said games scheduled for Saturday and Sunday were put on hold as well, "due to continued follow-up testing and contact tracing involving members of the Nationals organization."

The league did not indicate when it would potentially reschedule the games.

Ballparks across the United States welcomed back fans on Thursday for MLB's annual Opening Day, after last year's delayed and truncated 60-game regular season was played entirely to empty stands.